The present invention relates to nuclear well logging, and more particularly to statistical methods and apparatus for improving the quality of nuclear well logs.
In nuclear well logging, the data which is produced (e.g., count rates of specific events in one or more detectors) is highly statistical in nature. This therefore presents data enhancement problems unlike those in other types of well logging (e.g., electrical) in which the raw data signals are continuous. Continuous signals provide ample challenges for improving formation bed resolution, but do not suffer from the statistical variations which can plague nuclear logging.
Nuclear logging statistical variations (which become worse when, due to the tool or the formation, the count rates are low) are often a limiting factor in obtaining precise interpretations from nuclear logs. Digital log processing allows the application of enhanced filtering techniques. The goal is reduce statistical fluctuations while maintaining adequate response to formation changes. Alternatively, improved filtering permits increased logging speeds without degrading log quality.
More particularly, most nuclear well logs are based on counting rate measurements from gamma ray or neutron detectors. As indicated, low counting rates or high logging speeds increase the statistical uncertainties of the measurements. Averaging by analog or digital methods is normally employed to reduce these statistical fluctuations. However, excessive averaging degrades the observation of sudden changes in counting rate, such as might occur at formation bed boundaries.
In analog processing systems, resistor-capacitor combinations are selected with desired time constants to filter the counting rates. Variable time constants ("statistical dampeners") are often used that continuously adapt to varying count rates and reduce statistical fluctuations while maintaining formation bed resolution.
In digital computer logging systems, moving-average filters are often implemented in place of analog filters. An averaging interval can be selected to yield a reduction in statistical fluctuations that is similar to a particular analog time constant. However, some type of adaptive digital filtering is required to match the performance of analog statistical dampeners.
One type of adaptive digital filter can be provided by combining several digital filters known as "moving-average" filters. A moving-average filter may be understood as a running average of data samples. When a new sample is obtained, the oldest sample is deleted and a new average is calculated from the new sample plus the remaining samples. For nuclear logs, each data sample is normally a detector counting rate (counts per second) observed during a small time or depth increment. Filtering the data samples reduces statistical fluctuations that are often excessive between individual samples. However, both analog and digital filters usually have the undesirable effect of also smoothing out true formation changes. The longer the filter "length" (roughly equivalent to the length of the borehole interval being "averaged" or otherwise processed), the more likely that a thin formation will be missed, will not be fully resolved, will be misinterpreted as a thicker formation, or will otherwise be incorrectly logged.
A need therefore remains for a method and apparatus for use with nuclear borehole logs which can effectively reduce statistical variations in nuclear borehole logs while maintaining responsiveness to formation changes. Such a method and apparatus should be uncomplicated in concept and implementation, capable of efficient operation in real time logging situations, inexpensive, versatile, reliable, accurate, and readily applicable to the widest possible use with nuclear well logging techniques.